U.S. patent application number 14/909716 was filed with the patent office on 2016-06-30 for production method for porous polyimide resin film, porous polyimide resin film, and separator employing same.
The applicant listed for this patent is TOKYO OHKA KOGYO CO., LTD.. Invention is credited to Tsukasa SUGAWARA.
Application Number | 20160185932 14/909716 |
Document ID | / |
Family ID | 52461432 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160185932 |
Kind Code |
A1 |
SUGAWARA; Tsukasa |
June 30, 2016 |
PRODUCTION METHOD FOR POROUS POLYIMIDE RESIN FILM, POROUS POLYIMIDE
RESIN FILM, AND SEPARATOR EMPLOYING SAME
Abstract
A porous polyimide resin film having a high aperture ratio, and
a method for producing a porous polyimide film. The method includes
removing fine particles from a polyimide resin-fine particle
composite film to obtain a porous polyimide resin film by either
removing at least a part of a polyimide resin portion of the
polyimide resin-fine particle composite film prior to removing the
fine particles, or by removing at least a part of the porous
polyimide resin film subsequent to removing the fine particles.
Inventors: |
SUGAWARA; Tsukasa;
(Kawasaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOKYO OHKA KOGYO CO., LTD. |
Kawasaki-shi |
|
JP |
|
|
Family ID: |
52461432 |
Appl. No.: |
14/909716 |
Filed: |
August 6, 2014 |
PCT Filed: |
August 6, 2014 |
PCT NO: |
PCT/JP2014/070769 |
371 Date: |
February 2, 2016 |
Current U.S.
Class: |
429/246 ;
429/249; 521/183; 521/63 |
Current CPC
Class: |
H01M 2/14 20130101; H01M
10/0525 20130101; C08J 5/18 20130101; H01M 2/1653 20130101; C08J
2379/08 20130101; C08J 2201/0442 20130101; C08J 2201/044 20130101;
C08J 2207/00 20130101; H01M 2220/30 20130101; Y02E 60/10 20130101;
C08J 9/26 20130101; C08J 9/0066 20130101; H01M 2/145 20130101 |
International
Class: |
C08J 9/26 20060101
C08J009/26; H01M 2/14 20060101 H01M002/14; H01M 10/0525 20060101
H01M010/0525; H01M 2/16 20060101 H01M002/16 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2013 |
JP |
2013-165407 |
Mar 31, 2014 |
JP |
2014-073987 |
Claims
1. A method for producing a porous polyimide resin film comprising
removing fine particles from a polyimide resin-fine particle
composite film to thereby obtain a porous polyimide resin film,
wherein the method comprises either removing at least a part of a
polyimide resin portion of the polyimide resin-fine particle
composite film prior to removing the fine particles, or removing at
least a part of the porous polyimide resin film subsequent to
removing the fine particles.
2. The method for producing a porous polyimide resin film according
to claim 1, wherein the porous polyimide resin film after removing
the polyimide resin is 25 .mu.m in thickness and no greater than
120 seconds in Gurley permeability (JISP8117) with an amount of
permeated air being 100 ml.
3. The method for producing a porous polyimide resin film according
to claim 1, wherein the polyimide resin is removed by chemical
etching.
4. A porous polyimide resin film produced by the method according
to claim 1.
5. A separator composed of the porous polyimide resin film
according to claim 4.
6. A secondary battery comprising an electrolytic solution and the
separator according to claim 5 being disposed between a negative
electrode and a positive electrode.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
porous polyimide resin film, a porous polyimide resin film, and a
separator employing the film.
BACKGROUND ART
[0002] In recent years, because of a reduction in the size of
portable electronic devices and the development of hybrid
automobiles, electric automobiles, etc. for addressing
environmental problems such as an increase in atmospheric pollution
and carbon dioxide, secondary batteries having excellent
characteristics, such as high efficiency, high output, high energy
density, and light weight are needed. As such secondary batteries,
various secondary batteries are being researched and developed.
[0003] A lithium battery, which is a kind of secondary battery,
usually has a structure in which a space between a positive
electrode (cathode) and a negative electrode (anode) is filled with
a lithium salt, such as LiPF.sub.6, dissolved in an electrolytic
solution, for example, a non-aqueous organic solvent. Lithium
transition metal oxide is used as the positive electrode, and
lithium or carbon (graphite) is mainly used as the negative
electrode. The electrolytic solution has good ionic conductivity
and negligible electrical conductivity. During charging, lithium
ions move from the positive electrode to the negative electrode,
and during discharging, lithium ions move in the reverse
direction.
[0004] The positive electrode and the negative electrode of the
lithium battery are separated from each other with a separator of a
porous polymer film and are formed into a structure preventing
their direct electric contact. Accordingly, the separator for a
secondary battery is required to have various characteristics, such
as film thickness (thinness), mechanical strength, ionic
conductance (during containing of an electrolytic solution),
electric insulation, electrolytic solution resistance, electrolytic
solution-retaining property, and wettability. As the separators for
secondary batteries having these properties, microporous films made
of polyolefins, such as polyethylene and polypropylene, are
generally used. These microporous films have random pores at a
porosity of about 35% to 40% and are widely used as separators for
lithium secondary batteries having negative electrodes of
carbon.
[0005] However, these conventionally known separators are known to
deposit lithium metal on graphite negative electrodes due to
repeated charge and discharge cycles. Furthermore, the repetition
of charge and discharge of a battery is known to cause the growth
of dendrite lithium, resulting in a short circuit of the battery
and this problem needs to be solved (Patent Document 1).
Separately, it has been tried to use a polyimide having a high
heat-resistance and high safety in the separator (Patent Documents
2 and 3).
[0006] Patent Document 1: Japanese Unexamined Patent Application
(Translation of PCT Application), Publication No. 2010-537387
[0007] Patent Document 2: Japanese Unexamined Patent Application,
Publication No. 2011-111470
[0008] Patent Document 3: Japanese Unexamined Patent Application,
Publication No. 2012-107144
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] However, there was a problem in that the pores formed on the
conventional polyimide film did not necessarily have a sufficient
aperture ratio and could hinder movement of lithium ions, leading
to high internal resistance of the battery.
[0010] The present invention has been made in view of the above
circumstances, and an object thereof is to provide a method for
producing a porous polyimide resin film having a high aperture
ratio.
Means for Solving the Problems
[0011] The present inventors have discovered that the aperture
ratio of a porous polyimide resin film can be increased by removing
at least a part of a polyimide resin portion prior to formation of
pores or of the porous polyimide resin film subsequent to formation
of pores.
[0012] A first aspect of the present invention relates to a method
for producing a porous polyimide resin film comprising removing
fine particles from a polyimide resin-fine particles composite film
to thereby obtain a porous polyimide resin film, in which the
method comprises either removing at least a part of a polyimide
resin portion of the polyimide resin-fine particle composite film
prior to removing the fine particles, or removing at least a part
of the porous polyimide resin film subsequent to removing the fine
particles.
[0013] A second aspect of the present invention relates to a porous
polyimide resin film produced by the method according to the first
aspect of the present invention.
[0014] A third aspect of the present invention relates to a
separator composed of the porous polyimide resin film according to
the second aspect of the present invention.
[0015] A fourth aspect of the present invention relates to a
secondary battery comprising an electrolytic solution and the
separator according to the third aspect of the present invention
being disposed between a negative electrode and a positive
electrode.
Effects of the Invention
[0016] According to the present invention, a porous polyimide resin
film having a high aperture ratio can be produced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram schematically illustrating a polyimide
resin-fine particle composite film;
[0018] FIG. 2 is a diagram schematically illustrating an example in
which a part of a polyimide resin portion of the polyimide
resin-fine particle composite film has been removed;
[0019] FIG. 3 is a diagram schematically illustrating an example in
which a part of a polyimide resin portion of the polyimide
resin-fine particle composite film has been removed;
[0020] FIG. 4 is a diagram schematically illustrating a porous
polyimide resin film prior to a polyimide resin removing step;
[0021] FIG. 5 is a diagram schematically illustrating an example in
which a part of the porous polyimide resin film has been
removed;
[0022] FIG. 6 is a diagram schematically illustrating an example in
which a part of the porous polyimide resin film has been
removed;
[0023] FIG. 7 is a diagram illustrating a surface state of the
porous polyimide resin film after the polyimide resin removing step
by chemical etching;
[0024] FIG. 8 is a diagram illustrating a surface state of the
porous polyimide resin film after the polyimide resin removing step
by a physical method;
[0025] FIG. 9 is a diagram illustrating a negative electrode
surface of a secondary battery employing a separator according to
the present invention, after a charge/discharge test;
[0026] FIG. 10 is a diagram illustrating a negative electrode
surface of a secondary battery employing a general-purpose
polyethylene (PE) separator, after a charge/discharge test; and
[0027] FIG. 11 is a diagram illustrating a negative electrode
surface of a secondary battery employing a general-purpose
cellulose separator, after a charge/discharge test.
PREFERRED MODE FOR CARRYING OUT THE INVENTION
[0028] Embodiments of the present invention will now be described
in detail, but the present invention is not limited to the
following embodiments and can be implemented with appropriate
modifications within the purpose of the present invention.
[0029] The method for producing a polyimide resin film, which is
the first aspect of the present invention, comprises either
removing at least a part of a polyimide resin portion of the
polyimide resin-fine particle composite film prior to removing the
fine particles, or removing at least a part of the porous polyimide
resin film subsequent to removing the fine particles.
[0030] As a polyimide resin mentioned in the present Specification,
polyimide or polyamide imide can be exemplified.
[Production of Varnish]
[0031] Varnish can be produced by mixing a polyamide acid or
polyimide with an organic solvent in which fine particles are
dispersed in advance at an appropriate ratio, or by polymerizing a
tetracarboxylic dianhydride and a diamine into a polyamide acid in
an organic solvent in which fine particles are dispersed in
advance, or by further performing imidization into a polyimide. The
final viscosity thereof is preferably adjusted to 300 to 1500 cP
and more preferably in a range of 400 to 700 cP. The varnish having
a viscosity within this range can be formed into a uniform
film.
[0032] Resin fine particles and polyamide acid or polyimide or
polyamide imide can be blended into the above described varnish
such that a ratio of fine particles/polyimide resin is 1 to 3.5
(mass ratio) when a polyimide resin-fine particle composite film is
obtained by calcination (if calcination is optional, drying), and
the ratio of fine particles/polyimide resin is preferably 1.2 to 3
(mass ratio). Furthermore, the fine particles and the polyamide
acid or polyimide or polyamide imide may be blended so as to
provide the polyimide resin-fine particle composite film having a
volume ratio of the fine particles/polyimide of 1.5 to 4.5.
Moreover, the ratio of fine particles/polyimide resin of 1.8 to 3
(volume ratio) is more preferable. If a mass ratio or volume ratio
of fine particles/polyimide is not lower than the lower limit, a
film can have pores at an appropriate density as a separator; if
the mass ratio or volume ratio is not higher than the upper limit,
a film can be stably formed without causing problems such as an
increase in viscosity or cracking in the film. It should be noted
that values for percent by volume and volume ratios are values at
25.degree. C.
<Fine Particles>
[0033] Material for the fine particles used in the present
invention is not particularly limited, and can be any material that
is insoluble in the organic solvent to be used in the varnishes and
can be selectively removed after film formation. Examples of the
inorganic material include metal oxides such as silica (silicon
dioxide), titanium oxide, and alumina (Al.sub.2O.sub.3); examples
of the organic materials include high-molecular-weight olefins
(such as polypropylene and polyethylene) and organic polymer fine
particles (resin fine particles) such as polystyrenes, acrylic
resins (methyl methacrylate, isobutyl methacrylate, polymethyl
methacrylate (PMMA), etc.), epoxy resins, celluloses, polyvinyl
alcohols, polyvinyl butyrals, polyesters, and polyethers.
[0034] As preferable fine particles to be used for producing the
porous polyimide resin film, among inorganic materials, silica such
as colloidal silica can be exemplified. Among these, it is
preferable to choose monodisperse spherical silica particles in
order to form uniform and fine pores.
[0035] Resin fine particles used in the present invention can be
selected from general linear polymers and known depolymerizable
polymers, according to purpose without particular limitation. The
linear polymer usually has a molecular chain that is randomly
cleaved during thermal decomposition; and the depolymerizable
polymer is decomposed into monomers during thermal decomposition.
Both of them can be removed from the polyimide resin film by
decomposing into monomers, a low molecular weight substance, or to
CO.sub.2 during heating. The resin fine particles to be used
preferably have a decomposition temperature of 200.degree. C. to
320.degree. C. and more preferably 230.degree. C. to 260.degree. C.
A decomposition temperature of 200.degree. C. or higher allows
formation of a film even if the varnish contains a high boiling
point solvent, and broadens the selection of calcination conditions
of the polyimide resin. In addition, a decomposition temperature of
no higher than 320.degree. C. allows the resin fine particles to
disappear without thermally damaging the polyimide resin.
[0036] Among these depolymerizable polymers, homopolymers of methyl
methacrylate or isobutyl methacrylate (polymethyl methacrylate or
polyisobutyl methacrylate) having low thermal decomposition
temperature, or copolymers composed mainly of this are preferred
from the viewpoint of handleability during pore formation.
[0037] The fine particles to be used in the present invention
preferably have a high sphericity and a low particle diameter
distribution index. Fine particles satisfying these requirements
show excellent dispersibility in the varnish and can be used
without causing aggregation with one another. As the fine
particles, those having a particle diameter (average diameter) of,
for example, 100 to 2000 nm can be used. The fine particles
satisfying these requirements are preferable for providing pores
having uniform pore diameters in the porous film obtained by
removing the fine particles and, especially when the film is used
as a separator, homogenizing an electric field being applied.
[0038] In the production method described later, if a uncalcined
composite film is to be formed as a two-layered uncalcined
composite film, fine particles (B1) in the first varnish and fine
particles (B2) in the second varnish may be either the same or
different. In order to increase the density of the pores on the
side in contact with the substrate to be higher than that on the
other side, the fine particles (B1) preferably have a particle
diameter distribution index lower than or equal to that of the fine
particles (B2). Alternatively, the fine particles (B1) preferably
have a sphericity lower than or equal to that of the fine particles
(B2). In addition, the particle diameter (average diameter) of the
fine particles (B1) is preferably smaller than that of the fine
particles (B2). In particular, the fine particles (B1) preferably
have a particle diameter of 100 to 1000 nm (more preferably 100 to
600 nm), and the fine particles (B2) preferably have a particle
diameter of 500 to 2000 nm (more preferably 700 to 2000 nm). The
use of the fine particles (B1) having a particle diameter smaller
than that of the fine particles (B2) can give pores at a high
aperture proportion with a small variation in the aperture
proportion on the surface of the porous polyimide resin film being
obtained, and can increase the strength of the film compared to the
case of using fine particles having the same particle diameter as
that of the fine particles (B1) in the entire porous polyimide
resin film.
[0039] In the present invention, the varnish may further contain a
dispersant in addition to the fine particles, in order to uniformly
disperse the fine particles in the varnish. The addition of the
dispersant allows further uniform mixing of the polyamide acid,
polyimide, or polyamide imide with the fine particles and further
uniform dispersion of the fine particles in the molded or formed
precursor film. As a result, dense apertures are provided on the
surface of the finally formed porous polyimide resin, and the front
and rear surfaces can be efficiently interconnected with each other
to improve the air permeability of the film.
[0040] The dispersant used in the present invention is not
particularly limited and may be any known one. Examples of the
dispersant include, but are not limited to, anionic surfactants
such as salts of coconut fatty acid, salts of sulfonated castor
oil, lauryl sulfate, polyoxyalkylene allylphenyl ether sulfate,
alkylbenzenesulfonic acid, alkylbenzene sulfonate, alkyldiphenyl
ether disulfonate, alkylnaphthalene sulfonate, dialkyl
sulfosuccinate, isopropyl phosphate, polyoxyethylene alkyl ether
phosphate, and polyoxyethylene allylphenyl ether phosphate;
cationic surfactants such as oleylamine acetate, lauryl pyridinium
chloride, cetyl pyridinium chloride, lauryl trimethylammonium
chloride, stearyl trimethylammonium chloride, behenyl
trimethylammonium chloride, and didecyl dimethylammonium chloride;
amphoteric surfactants such as coconut alkyl dimethylamine oxide,
fatty acid amide propyl dimethyl amine oxide, alkyl polyaminoethyl
glycine hydrochloride, amide betaine surfactant, alanine
surfactant, and lauryl iminodipropionic acid; polyoxyalkylene
primary alkyl ether or polyoxyalkylene secondary alkyl ether
nonionic surfactants such as polyoxyethylene octyl ether,
polyoxyethylene decyl ether, polyoxyethylene lauryl ether,
polyoxyethylene laurylamine, polyoxyethylene oleylamine,
polyoxyethylene polystyryl phenyl ether, and polyoxyalkylene
polystyryl phenyl ether; other polyoxyalkylene nonionic surfactants
such as polyoxyethylene dilaurate, polyoxyethylene laurate,
polyoxyethylenated castor oil, polyoxyethylenated hydrogenated
castor oil, sorbitan laurate, polyoxyethylene sorbitan laurate, and
fatty acid diethanolamide; fatty acid alkyl esters such as octyl
stearate and trimethylolpropane tridecanoate; and polyether polyols
such as polyoxyalkylene butyl ether, polyoxyalkylene oleyl ether,
and trimethylol propane tris (polyoxyalkylene) ether. These
dispersants may be used as a mixture of two or more thereof.
<Polyamide Acid>
[0041] As the polyamide acid used in the present invention, one
obtained by polymerizing any tetracarboxylic dianhydride and
diamine can be used without particular limitation. The amounts of
the tetracarboxylic dianhydride and the diamine are not
particularly limited, and the amount of the diamine is preferably
0.50 to 1.50 mol, more preferably 0.60 to 1.30 mol, and most
preferably 0.70 to 1.20 mol, based on 1 mol of the tetracarboxylic
dianhydride.
[0042] The tetracarboxylic dianhydride can be appropriately
selected from tetracarboxylic dianhydrides that have been
conventionally used as raw materials for synthesizing polyamide
acids. The tetracarboxylic dianhydride may be an aromatic
tetracarboxylic dianhydride or an aliphatic tetracarboxylic
dianhydride, but from the viewpoint of the heat resistance of the
resulting polyimide resin, an aromatic tetracarboxylic dianhydride
is preferably used. The tetracarboxylic dianhydrides may be used in
a combination of two or more thereof.
[0043] Preferred examples of the aromatic tetracarboxylic
dianhydride include pyromellitic dianhydride,
1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,
bis(2,3-dicarboxyphenyl)methane dianhydride,
bis(3,4-dicarboxyphenyl)methane dianhydride,
3,3',4,4'-biphenyltetracarboxylic dianhydride,
2,3,3',4'-biphenyltetracarboxylic dianhydride,
2,2,6,6-biphenyltetracarboxylic dianhydride,
2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,
2,2-bis(2,3-dicarboxyphenyl)propane dianhydride,
2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane
dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,
3-hexafluoropropane dianhydride,
3,3',4,4'-benzophenonetetracarboxylic dianhydride,
bis(3,4-dicarboxyphenyl)ether dianhydride,
bis(2,3-dicarboxyphenyl)ether dianhydride,
2,2',3,3'-benzophenonetetracarboxylic dianhydride,
4,4-(p-phenylenedioxy)diphthalic dianhydride,
4,4-(m-phenylenedioxy)diphthalic dianhydride,
1,2,5,6-naphthalenetetracarboxylic dianhydride,
1,4,5,8-naphthalenetetracarboxylic dianhydride,
2,3,6,7-naphthalenetetracarboxylic dianhydride,
1,2,3,4-benzenetetracarboxylic dianhydride,
3,4,9,10-perylenetetracarboxylic dianhydride,
2,3,6,7-anthracenetetracarboxylic dianhydride,
1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bisphthalic
anhydride fluorene, and 3,3',4,4'-diphenylsulfonetetracarboxylic
dianhydride. Examples of the aliphatic tetracarboxylic dianhydride
include ethylenetetracarboxylic dianhydride, butanetetracarboxylic
dianhydride, cyclopentanetetracarboxylic dianhydride,
cyclohexanetetracarboxylic dianhydride,
1,2,4,5-cyclohexanetetracarboxylic dianhydride, and
1,2,3,4-cyclohexanetetracarboxylic dianhydride. Among these,
3,3',4,4'-biphenyl tetra carboxylic dianhydride is preferable from
the viewpoint of price, availability, and the like. These
tetracarboxylic dianhydrides may be used alone or as a mixture of
two or more thereof.
[0044] The diamine can be appropriately selected from diamines that
have been conventionally used as raw materials for synthesizing
polyamide acids. The diamine can be either an aromatic diamine or
an aliphatic diamine; however, an aromatic diamine is preferable
from the viewpoint of heat resistance of the resulting polyimide
resin. These diamines may be used in a combination of two or more
thereof.
[0045] Examples of the aromatic diamine include diamino compounds
having one phenyl group or about two to ten phenyl groups.
Specifically, examples of the aromatic diamine include
phenylenediamines and their derivatives, diaminobiphenyl compounds
and their derivatives, diaminodiphenyl compounds and their
derivatives, diaminotriphenyl compounds and their derivatives,
diaminonaphthalenes and their derivatives, aminophenylaminoindanes
and their derivatives, diaminotetraphenyl compounds and their
derivatives, diaminohexaphenyl compounds and their derivatives, and
cardo-type fluorenediamine derivatives.
[0046] The phenylenediamines are, for example, m-phenylenediamine
and p-phenylenediamine. The phenylenediamine derivatives are
diamines to which alkyl groups, such as a methyl group or an ethyl
group, are bound, such as 2,4-diaminotoluene and
2,4-triphenylenediamine.
[0047] The diaminodiphenyl compounds are obtained by linkage of two
aminophenyl groups at their phenyl groups. For example, the
diaminodiphenyl compounds are 4,4'-diaminobiphenyl and
4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.
[0048] Alternatively, the diaminodiphenyl compounds are obtained by
linkage of two aminophenyl groups at their phenyl groups via
another group. The linkage is, for example, an ether linkage, a
sulfonyl linkage, a thioether linkage, a linkage of an alkylene or
its derivative group, an imino linkage, an azo linkage, a phosphine
oxide linkage, an amide linkage, or an ureylene linkage. The
alkylene linkage is a linkage of an alkylene having about 1 to 6
carbon atoms, and its derivative group is an alkylene group whose
one or more hydrogen atoms have been replaced by, for example,
halogen atoms.
[0049] Examples of the diaminodiphenyl compounds include
3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether,
4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone,
3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone,
3,3'-diaminodiphenyl methane, 3,4'-diaminodiphenyl methane,
4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfide,
3,3'-diaminodiphenyl ketone, 3,4'-diaminodiphenyl ketone,
2,2-bis(p-aminophenyl)propane, 2,2'-bis(p-aminophenyl)
hexafluoropropane, 4-methyl-2,4-bis(p-aminophenyl)-1-pentene,
4-methyl-2,4-bis(p-aminophenyl)-2-pentene, iminodianiline,
4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphine
oxide, 4,4'-diaminoazobenzene, 4,4'-diaminodiphenylurea,
4,4'-diaminodiphenylamide, 1,4-bis(4-aminophenoxy)benzene,
1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene,
4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl]
sulfone, bis[4-(3-aminophenoxy)phenyl] sulfone,
2,2-bis[4-(4-aminophenoxy)phenyl] propane, and
2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.
[0050] Among these, p-phenylenediamine, m-phenylenediamine,
2,4-diamino toluene, and 4,4'-diamino diphenyl ether are preferable
from the viewpoint of price, availability, and the like.
[0051] The diaminotriphenyl compound is formed by linkage of two
aminophenyl groups and one phenylene group, all of which are linked
to each other through another group, the another group being
selected from the same groups as in the diaminodiphenyl compounds.
Examples of the diaminotriphenyl compounds include
1,3-bis(m-aminophenoxy)benzene, 1,3-bis(p-aminophenoxy)benzene, and
1,4-bis(p-aminophenoxy)benzene.
[0052] Examples of the diaminonaphthalenes include
1,5-diaminonaphthalene and 2,6-diaminonaphthalene.
[0053] Examples of the aminophenylaminoindanes include 5- or
6-amino-1-(p-aminophenyl)-1,3,3-trimethylindane.
[0054] Examples of the diaminotetraphenyl compounds include
4,4'-bis(p-aminophenoxy)biphenyl,
2,2'-bis[p-(p'-aminophenoxy)phenyl]propane,
2,2'-bis[p-(p'-aminophenoxy)biphenyl]propane, and
2,2'-bis[p-(m-aminophenoxy)phenyl] benzophenone.
[0055] Examples of the cardo-type fluorenediamine derivatives
include 9,9-bisanilinefluorene.
[0056] The aliphatic diamine has, for example, about 2 to 15 carbon
atoms, and specifically, examples thereof include
pentamethylenediamine, hexamethylenediamine, and
heptamethylenediamine.
[0057] The aliphatic diamine may be a compound having at least one
substituent selected from the group consisting of halogen atoms and
methyl, methoxy, cyano, and phenyl groups for hydrogen atoms of the
diamine.
[0058] The polyamide acid to be used in the present invention may
be produced by any method without particular limitation, and can be
produced by a known method, for example by reaction of an acid with
a diamine component in organic solvent.
[0059] The reaction of a tetracarboxylic dianhydride and a diamine
is usually performed in an organic solvent. The organic solvent to
be used for the reaction of a tetracarboxylic dianhydride and a
diamine may be any organic solvent that can dissolve the
tetracarboxylic dianhydride and the diamine without reacting with
the tetracarboxylic dianhydride and the diamine. The organic
solvent may be a single solvent or a mixture of two or more
solvents.
[0060] Examples of the organic solvent used for reaction of
tetracarboxylic dianhydride with diamine include:
nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone,
N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide,
N,N-diethylformamide, N-methylcaprolactam, and
N,N,N',N'-tetramethylurea; lactone-based polar solvents such as
.beta.-propiolactone, .gamma.-butyrolactone, .gamma.-valerolactone,
.gamma.-valerolactone, .gamma.-caprolactone and
.epsilon.-caprolactone; dimethylsulfoxide; acetonitrile; fatty acid
esters such as ethyl lactate and butyl lactate; ethers such as
diethyleneglycol dimethylether, diethyleneglycol diethylether,
dioxane, tetrahydrofuran, methyl cellosolve acetate and ethyl
cellosolve acetate; and phenol solvents such as cresols. These
organic solvents may be used alone or as a mixture of two or more
thereof. Among these, a combination of the nitrogen-containing
polar solvent with the lactone-based polar solvent is preferable.
The amount of the organic solvent is not particularly limited but
is desirably such an amount that the content of the resulting
polyamide acid is 5% to 50% by mass.
[0061] Among these organic solvents, from the viewpoint of the
solubility of the resulting polyamide acid, preferred are
nitrogen-containing polar solvents, such as N-methyl-2-pyrrolidone,
N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide,
N,N-diethylformamide, N-methylcaprolactam, and
N,N,N',N'-tetramethylurea. Alternatively, from the viewpoint of
film forming property and the like, the organic solvent can be a
blended solvent comprising lactone-based polar solvent such as
.gamma.-butyrolactone, which is preferably 1 to 20% by mass and
more preferably 5 to 15% by mass with respect to the entire amount
of organic solvent.
[0062] The polymerization temperature is usually -10.degree. C. to
120.degree. C. and preferably 5.degree. C. to 30.degree. C. The
polymerization time varies depending on the raw material
composition and is usually 3 to 24 hours (hr). The polyamide acid
solution prepared under such conditions preferably has an intrinsic
viscosity of 1000 to 100000 centipoises (cP), more preferably in a
range of 5000 to 70000 cP.
<Polyimide>
[0063] The polyimide used in the present invention can be any known
polyimide, without restriction by its structure and molecular
weight, as long as the polyimide is soluble in the organic solvent
to be used in the varnish according to the present invention. The
side chain of the polyimide may have a condensable functional
group, such as a carboxy group, or a functional group enhancing the
cross-linking reaction during calcination.
[0064] In order to make the polyimide soluble in an organic
solvent, it is effective to use a monomer for introducing a
flexible bend structure into the main chain, for example, to use an
aliphatic diamine, such as ethylenediamine, hexamethylenediamine,
1,4-diaminocyclohexane, 1,3-diaminocyclohexane, or
4,4'-diaminodicyclohexylmethane; an aromatic diamine, such as
2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine,
3,3'-dimethoxybenzidine, or 4,4'-diaminobenzanilide; a
polyoxyalkylenediamine, such as polyoxyethylenediamine,
polyoxypropylenediamine, or polyoxybutyrenediamine; a
polysiloxanediamine; 2,3,3',4'-oxydiphthalic anhydride,
3,4,3',4'-oxydiphthalic anhydride, or
2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3',4,4'-tetracarboxylic
dianhydride. It is also effective to use a monomer containing a
functional group for improving the solubility in an organic
solvent, for example, to use a fluorinated diamine, such as
2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl or
2-trifluoromethyl-1,4-phenylenediamine. Furthermore, in addition to
the monomer for improving the solubility of the polyimide, a
monomer that is mentioned in the paragraph describing the polyamide
acid may be used within a range that does not inhibit the
solubility.
[0065] The polyimide soluble in an organic solvent to be used in
the present invention may be produced by any method and, for
example, can be produced by a known method of, for example,
chemically imidizing or thermally imidizing a polyamide acid and
dissolving the imidized polyamide in an organic solvent. Examples
of such polyimides include aliphatic polyimide (full-aliphatic
polyimides) and aromatic polyimides, and aromatic polyimides are
preferred. The aromatic polyimide may be one prepared by a thermal
or chemical ring-closing reaction of a polyamide acid having
repeating units represented by Formula (1) or one prepared by
dissolving a polyimide having repeating units represented by
Formula (2) in a solvent. In the formulae, Ar represents an aryl
group.
##STR00001##
<Polyamide Imide>
[0066] The polyamide imide used in the present invention can be any
known polyamide imide, without restriction by its structure and
molecular weight, as long as the polyamide imide is soluble in the
organic solvent to be used in the varnish according to the present
invention. The side chain of the polyamide imide may have a
condensable functional group, such as a carboxy group, or a
functional group enhancing a cross-linking reaction etc. during
calcination.
[0067] In addition, as the polyamide imide used in the present
invention, polyamide imide obtained by reacting any trimellitic
anhydride with diisocyanate, and polyamide imide obtained by
imidizing a precursor polymer obtained as a result of reaction of a
reactive derivative of any trimellitic anhydride with diamine can
be used without particular limitation.
[0068] As the above mentioned any trimellitic anhydride or reactive
derivative thereof, for example: trimellitic anhydride; trimellitic
anhydride halides such as trimellitic anhydride chloride;
trimellitic anhydride esters; and the like can be exemplified.
[0069] As the above mentioned any diisocyanate, for example,
meta-phenylene diisocyanate, p-phenylene diisocyanate, o-tolidine
diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate,
4,4'-oxybis (phenyl isocyanate), 4,4'-diphenylmethane diisocyanate,
bis[4-(4-isocyanate phenoxy) phenyl] sulfone,
2,2'-bis[4-(4-isocyanate phenoxy) phenyl] propane, 2,4-tolylene
diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane
diisocyanate, 3,3'-dimethyldiphenyl-4,4'-diisocyanate,
3,3'-diethyldiphenyl-4,4'-diisocyanate, isophorone diisocyanate,
hexamethylene diisocyanate, 4,4'-dicyclohexyl methane diisocyanate,
m-xylene diisocyanate, p-xylene diisocyanate, naphtalen
diisocyanate, and the like can be exemplified.
[0070] As the above mentioned any diamine, those exemplified above
in the description with regard to the polyamide acid can be
exemplified.
<Organic Solvent>
[0071] The organic solvent used in the varnish is not particularly
limited as long as the solvent can dissolve a resin composed of
polyamide acid and/or polyimide while not dissolving the fine
particles; and can include those exemplified as the solvent used
for reaction of tetracarboxylic dianhydride with diamine. The
solvent can be used alone or in a combination of two or more
types.
[0072] The content of the blended solvent (S) in all components of
the varnish is preferably 50 to 95% by mass, and more preferably 60
to 85% by mass. The solid content concentration of the varnish is
preferably 5 to 50% by mass, and more preferably 15 to 40% by
mass.
[0073] In the production method described later, if an uncalcined
composite film is to be formed as a two-layered uncalcined
composite film, a volume ratio of polyamide acid, polyimide or
polyamide imide (A1) to the fine particles (B1) in the first
varnish is preferably 19:81 to 45:65. If the volume of the fine
particles is at least 65 with respect to the total volume of the
varnish being 100, the particles are uniformly dispersed; and if
the volume is 81 or less, the particles are dispersed without
causing aggregation with one another, allowing uniform formation of
pores on the surface of the polyimide resin film on the substrate
side. In addition, a volume ratio of polyamide acid, polyimide or
polyamide imide (A2) to the fine particles (B2) in the second
varnish is preferably 20:80 to 50:50. If the volume of the fine
particles is at least 50 with respect to the total volume being
100, the particles themselves are uniformly dispersed; and if the
volume is 80 or less, the particles are not aggregated and no
cracking is caused on the surface, resulting in stable formation of
a porous polyimide resin film having superior electrical
characteristics.
[0074] As to the above described volume ratio, it is preferable
that the second varnish is lower in the fine particle content than
the first varnish; by satisfying this condition, even if the
polyamide acid, polyimide or polyamide imide contains a large
amount of fine particles, the strength and flexibility of the
uncalcined composite film, the polyimide resin-fine particle
composite film, and the porous polyimide resin film can be secured.
In addition, the lower content ratio of the fine particles in the
layer can reduce the cost of producing the film.
[0075] In addition to the above mentioned components, known
components such as an antistatic agent, a fire retardant, a
chemical imidizing agent, a condensation agent, a release agent, a
superficial regulator, etc. can be added as necessary for static
prevention, fire resistance, releasability, applicability, etc.
[Production of Unburned Composite Film]
[0076] The uncalcined composite film containing polyamide acid or
polyimide resin as well as the fine particles is formed by:
applying the above described varnish onto a substrate; and drying
under ordinary pressure or under vacuum at 50 to 100.degree. C.
(preferably at 0 to 50.degree. C.), more preferably under ordinary
pressure at 60 to 95.degree. C. (further preferably at 65 to
90.degree. C.). A releasing layer can also be provided on the
substrate.
[0077] The releasing layer can be provided by applying a releasing
agent onto the substrate and drying or baking. The releasing agent
used here may be a known releasing agent, such as an alkylphosphate
ammonium salt-based or fluorine-based agent or silicon, without
particular restriction. Upon peeling of the dried uncalcined
composite film containing polyamide acid or polyimide resin as well
as the fine particles from the substrate, a slight amount of the
releasing agent remains on the peeled surface of the uncalcined
composite film. The releasing agent thus remaining would have
substantial adverse effects on wettability and electrical
characteristics of the film surface, and is therefore preferably
removed.
[0078] Accordingly, the uncalcined composite film peeled from the
substrate is preferably washed with an organic solvent and the
like. The washing method can be selected from known methods, such
as a method in which the film is immersed in a washing solution and
is then taken out, a method of shower washing, and the like.
Furthermore, in order to dry the uncalcined composite film thus
washed, known methods such as air drying at ambient temperature,
heating in a thermostatic bath up to an appropriate temperature,
and the like can be applied without limitation to the uncalcined
composite film thus washed. For example, a method of fixing ends of
the uncalcined composite film to a frame or the like made of
stainless steel to thereby prevent deformation can be employed.
[0079] Alternatively, when a substrate is directly used without
being provided with a releasing layer in formation of the
uncalcined composite film, the steps of forming the releasing layer
and washing the uncalcined composite film can be omitted.
[0080] Alternatively, in a case of forming a two-layered uncalcined
composite film, a first uncalcined composite film of 1 to 5 .mu.m
in thickness is formed by: applying the above described first
varnish onto a substrate as is; and drying under ordinary pressure
or under vacuum at 0 to 100.degree. C. (preferably at 0 to
90.degree. C.), more preferably under ordinary pressure at 10 to
100.degree. C. (further preferably at 10 to 90.degree. C.)
[0081] Subsequently, a second uncalcined composite film of 5 to 30
.mu.m in thickness is formed by: applying the above described
second varnish onto the first uncalcined composite film thus
formed; and drying similarly at 0 to 80.degree. C. (preferably at 0
to 50.degree. C.), more preferably under ordinary pressure at 10 to
80.degree. C. (further preferably at 10 to 30.degree. C.) to
thereby obtain the two-layered uncalcined composite film.
[Production of Polyimide Resin-Fine Particle Composite Film
(Calcining Step)]
[0082] The above described uncalcined composite film (or the
two-layered uncalcined composite film, the same applies hereafter)
after drying can be subjected to post-treatment by heating
(calcination) to obtain a composite film (polyimide resin-fine
particle composite film) composed of the polyimide resin and the
fine particles. In a case in which the varnish contains polyamide
acid, imidization is preferably completed in the calcining step. It
should be noted that the calcining step is optional. Especially in
a case in which polyamide or polyamide imide is used in the
varnish, the calcining step can be omitted.
[0083] The calcination temperature varies depending on the
structure of polyamide acid or polyimide resin contained in the
uncalcined composite film and the presence or absence of a
condensing agent, and is preferably 120.degree. C. to 375.degree.
C. and more preferably 150.degree. C. to 350.degree. C.
[0084] Calcination is not necessarily clearly separated from the
drying step, and in a case of calcining at 375.degree. C., for
example, a method of increasing the temperature from room
temperature to 375.degree. C. over 3 hours and then holding at
375.degree. C. for 20 minutes; or a stepwise drying-thermal
imidization method of increasing the temperature stepwise by
50.degree. C. from room temperature to 375.degree. C. (holding the
temperature of each step for 20 minutes) and finally holding at
375.degree. C. for 20 minutes. Here, a method of fixing ends of the
uncalcined composite film to a frame or the like made of stainless
steel to thereby prevent deformation can be employed.
[0085] The thickness of the resulting polyimide resin-fine particle
composite film can be determined by, for example, measuring the
thicknesses of a plurality of positions by a micrometer or the like
and averaging the thicknesses. Preferred average film thickness
varies depending on the purpose of the polyimide resin-fine
particle composite film or the porous polyimide resin film,
however, film thickness can be at least 1 .mu.m, preferably 5 to
500 .mu.m, and more preferably 10 to 100 .mu.m, in the use as a
separator for example. [Fine particle removing step (Porosification
of polyimide resin-fine particle composite film)]
[0086] The porous polyimide resin film can be produced with high
reproducibility by selecting an appropriate method for removing the
fine particles from the polyimide resin-fine particle composite
film. For example, in a case of employing silica as the fine
particles, the polyimide-fine particle composite film can be made
porous by dissolving and removing the silica with a
low-concentration hydrogen fluoride solution (HF). In addition, in
a case in which the fine particles are resin fine particles, the
resin fine particles can be decomposed and removed by heating at a
temperature equal to or greater than the above specified thermal
decomposition temperature of the resin fine particles and lower
than the thermal decomposition temperature of the polyimide
resin.
[Polyimide Resin Removing Step]
[0087] The method for producing a polyimide resin film of the
present invention comprises a polyimide resin removing step of
either removing at least a part of a polyimide resin portion of the
polyimide resin-fine particle composite film prior to the fine
particle removing step, or removing at least a part of the porous
polyimide resin film subsequent to the fine particle removing
step.
[0088] First, a polyimide resin removing step of removing at least
a part of a polyimide resin portion of the polyimide resin-fine
particle composite film prior to the fine particle removing step is
described.
[0089] FIG. 1 is a diagram schematically illustrating a polyimide
resin-fine particle composite film. Fine particles 1 are dispersed
in a polyimide resin portion 2 to thereby form the polyimide
resin-fine particle composite film. In the vicinity of a surface of
the polyimide resin-fine particle composite film, the polyimide
resin portion 2 covers a part of or entire fine particles 1. It
should be noted that, since FIG. 1 is a schematic diagram, the fine
particles 1 are illustrated in substantially the same diameter;
however, the present invention is not limited thereto and the
particle diameters of the fine particles 1 can have a
distribution.
[0090] The above mentioned "removing at least a part of a polyimide
resin portion prior to the fine particles removing step" means
removing any part of the polyimide resin portion 2 in FIG. 1. This
means, for example, making configuration as in FIG. 2 or FIG. 3;
however, the present invention is not limited thereto. FIG. 2
illustrates a case in which a relatively small amount of polyimide
resin portion is removed; and FIG. 3 illustrates a case in which a
larger amount of polyimide resin portion is removed. In the
polyimide resin removing step, the fine particles can also be
partially removed.
[0091] By removing a part of the polyimide resin portion 2, when
the fine particles are removed and pores are formed in the
subsequent fine particles removing step, the aperture ratio of the
porous polyimide resin film as a final product can be increased,
compared to a case of not removing the polyimide resin portion.
[0092] Next, a polyimide resin removing step of removing at least a
part of the porous polyimide resin film subsequent to the fine
particle removing step is described.
[0093] FIG. 4 is a diagram schematically illustrating the porous
polyimide resin film immediately after the fine particle removing
step. Pores 3 formed by disappearance of the fine particles are
distributed in the porous polyimide resin film 4. Similarly to FIG.
1, since FIG. 4 is a schematic diagram, the pores 3 are illustrated
in substantially the same diameter; however, the present invention
is not limited thereto and the diameters of the pores 3 can have a
distribution.
[0094] "Removing at least a part of the porous polyimide resin film
subsequent to the fine particles removing step" means removing any
part of the porous polyimide resin film 4 in FIG. 4. This means,
for example, making configuration as in FIG. 5 or FIG. 6; however,
the present invention is not limited thereto. FIG. 5 illustrates a
case in which a relatively small amount of porous polyimide resin
film is removed; and FIG. 6 illustrates a case in which a larger
amount of porous polyimide resin film is removed. Alternatively,
the pores can be made so as to interconnect with each other, by
removing the porous polyimide resin film between adjacent
pores.
[0095] By removing a part of the porous polyimide resin film, the
aperture ratio of the porous polyimide resin film as a final
product can be increased, compared to a case of not removing.
[0096] The above mentioned step of removing at least a part of the
polyimide resin portion, or the step of removing at least a part of
the porous polyimide resin film can be performed by a normal
chemical etching method, physical removal method, or a combination
thereof.
[0097] As the chemical etching method, a treatment with chemical
etching solution such as inorganic alkali solution or organic
alkali solution can be exemplified. The inorganic alkali solution
is preferable. As the inorganic alkali solution: hydrazine solution
containing hydrazine hydrate and ethylenediamine; solution of
alkali metal hydroxides such as potassium hydroxide, sodium
hydroxide, sodium carbonate, sodium silicate, sodium metasilicate;
ammonia solution; etching solution composed primarily of
hydroxylation alkali, hydrazine and 1,3-dimethyl-2-imidazolidinone
can be exemplified. As the organic alkali solution: primary amines
such as ethylamine and n-propylamine; secondary amines such as
diethylamine and di-n-butylamine; tertiary amines such as
triethylamine and methyldiethylamine; alcohol amines such as
dimethylethanolamine and triethanolamine; quaternary ammonium salts
such as tetramethylammonium hydroxide and tetraethylammonium
hydroxide; and alkaline solution of cyclic amines such as pyrrol
and piperidine, can be exemplified.
[0098] Pure water and alcohols can be selected as appropriate as a
solvent for the above mentioned solutions. A solution with an
appropriate amount of surfactant can also be used. Alkali
concentration is for example 0.01 to 20% by mass.
[0099] As the physical method, for example: dry etching by plasma
(oxygen, argon, etc.), corona discharge and the like; and a method
of dispersing abrasives (e.g., alumina (hardness 9)) in liquid and
spraying onto a surface of the polyimide resin-fine particle
composite film or the porous polyimide resin film at a rate of 30
to 100 m/s to treat the surface of the polyimide resin-fine
particle composite film or the porous polyimide resin film can be
employed.
[0100] The above described method is applicable to either of the
polyimide resin removing steps prior to and subsequent to the fine
particle removing step, and is therefore preferable. It should be
noted that, in a case of performing chemical etching after the fine
particle removing step, the aperture ratio can be increased by
increasing the size of interconnected pores (pores that are formed
where the fine particles were adjacent) in the porous polyimide
resin film.
[0101] On the other hand, as a physical method that can be employed
only in the polyimide resin removing step subsequent to the fine
particle removing step, a method of: pressure bonding a target
surface onto a mount film (for example, a polyester film such as a
PET film) being wetted with liquid; drying or not drying; and then
peeling the porous polyimide resin film from the mount film can
also be employed. Due to liquid surface tension or electrostatic
adhesive force, a porous polyimide resin film is torn off from a
mount film under conditions in which only the face of the porous
polyimide resin film was left on the mount film. In this method,
the configuration of the porous polyimide resin film is as
illustrated by the schematic diagram of FIG. 5.
[Porous Polyimide Resin Film]
[0102] The porous polyimide resin film produced by the above
described production method according to the present invention is
preferable for having a further increased aperture ratio, and, if
employed as a separator in a lithium-ion battery for example,
allowing downsizing of the inside of the battery.
[0103] The aperture ratio of the porous polyimide resin film can be
evaluated by obtaining Gurley permeability for a thickness of 25
.mu.m, in other words the number of seconds required for 100 ml air
permeating the film, according to JISP8117.
[0104] The Gurley permeability of the porous polyimide resin film
of the present invention is preferably no greater than 120 seconds,
more preferably no greater than 100 seconds, and most preferably no
greater than 80 seconds. Since smaller is better, a lower limit is
not particularly defined; however, in consideration of
handleability of the porous polyimide resin film, at least 30
seconds. The Gurley permeability of no greater than 120 seconds
indicates sufficiently high ion permeability, and the film is
preferable for use as a separator in a lithium-ion battery.
[Use of Porous Polyimide Resin Film]
[0105] The porous polyimide resin film of the present invention can
be used as the separator for a secondary battery, such as a nickel
cadmium battery, a nickel hydrogen battery, a lithium ion secondary
battery, and a lithium metal secondary battery and is particularly
preferably used as the porous separator for a lithium ion secondary
battery. Furthermore, the porous polyimide resin film produced by
the production method of the present invention can also be used as
the separator of a secondary battery, a fuel cell electrolyte film,
a film for separating a gas or liquid, or a low dielectric constant
material.
[Secondary Battery]
[0106] A secondary battery according to the fourth aspect of the
present invention is characterized in comprising an electrolytic
solution and the separator according to the third aspect being
disposed between a negative electrode and a positive electrode.
[0107] The secondary battery of the present invention may be of any
type and may have any configuration. The present invention can be
employed with no particular limitation in known secondary batteries
such as nickel cadmium and nickel hydrogen batteries as well as a
lithium ion secondary battery, as long as the battery has a
configuration in which a battery element including a positive
electrode, a separator, and a negative electrode being laminated in
this order so as to satisfy the above-described requirements is
impregnated with an electrolytic solution and encapsulated in an
outer package.
[0108] The negative electrode of the secondary battery of the
present invention can have a structure in which a negative
electrode mixture composed of a negative electrode active material,
a conductive auxiliary agent, and a binder is formed on a
collector. For example, as the negative electrode active material,
cadmium hydroxide can be used in nickel cadmium batteries, and a
hydrogen-occlusion alloy can be used in nickel hydrogen batteries.
In lithium ion secondary batteries, a material that can be
electrochemically doped with lithium can be employed. Examples of
such active materials include carbon materials, silicon, aluminum,
tin, and Wood's metal.
[0109] Examples of the conductive auxiliary agent constituting the
negative electrode include carbon materials such as acetylene black
and Ketjen black. The binder is composed of an organic polymer, and
examples thereof include polyvinylidene fluoride and carboxymethyl
cellulose. The collector can be, for example, copper foil,
stainless steel foil, or nickel foil.
[0110] The positive electrode can have a structure in which a
positive electrode mixture composed of a positive electrode active
material, a conductive auxiliary agent, and a binder is formed on a
collector. For example, the positive electrode active material can
be nickel hydroxide in nickel cadmium batteries and can be nickel
hydroxide or nickel oxyhydroxide in nickel hydrogen batteries.
Meanwhile, in lithium ion secondary batteries, examples of the
positive electrode active material include lithium-containing
transition metal oxides, specifically, LiCoO.sub.2, LiNiO.sub.2,
LiMn.sub.0.5Ni.sub.0.5O.sub.2,
LiCo.sub.1/3Ni.sub.1/3Mn.sub.1/3O.sub.2, LiMn.sub.2O.sub.4,
LiFePO.sub.4, LiCo.sub.0.5Ni.sub.0.5O.sub.2, and
LiAl.sub.0.25Ni.sub.0.75O.sub.2. Examples of the conductive
auxiliary agent include carbon materials such as acetylene black
and Ketjen black. The binder is an organic polymer, and examples
thereof include polyvinylidene fluoride. The collector can be, for
example, aluminum foil, stainless steel foil, or titanium foil.
[0111] The electrolytic solution in, for example, a nickel cadmium
battery or a nickel hydrogen battery is a potassium hydroxide
solution. The electrolytic solution in a lithium ion secondary
battery is composed by dissolving a lithium salt in a non-aqueous
solvent. Examples of the lithium salt include LiPF.sub.6,
LiBF.sub.4 and LiClO.sub.4. Examples of the non-aqueous solvent
include propylene carbonate, ethylene carbonate, dimethyl
carbonate, diethyl carbonate, ethyl methyl carbonate,
g-butyrolactone, and vinylene carbonate. These solvents may be used
alone or as a mixture.
[0112] Examples of the outer package material include metal cans
and aluminum laminate packs. The shape of the battery is, for
example, a rectangular shape, a cylindrical shape, or a coin shape;
however, the separator of the present invention can be suitably
applied to any shape.
EXAMPLES
[0113] The present invention will now be more specifically
described with reference to Examples, but the scope of the present
invention is not limited to the following examples.
[0114] In Examples and Comparative Examples, tetracarboxylic
dianhydride, diamine, polyamide acid, organic solvent, dispersant,
and fine particles described below were used. It should be noted
that the varnishes were prepared such that the final solid content
concentration was 30% by mass. [0115] Tetracarboxylic dianhydride:
pyromellitic dianhydride [0116] Diamine: 4,4'-diaminodiphenylether
[0117] Polyamide acid: reaction product of pyromellitic dianhydride
with 4,4'-diaminodiphenylether [0118] Polyamide imide: polyamide
containing trimellitic anhydride and o-tolidine diisocyanate as
polymerization components (Mw: approx. 30,000) [0119] Organic
solvent (1): blended solvent of N,N-dimethylacetamide and
.gamma.-butyrolactone (90:10 in mass ratio) [0120] Organic solvent
(2): blended solvent of N-methyl-2-pyrrolidone and
N,N-dimethylacetamide (70:30 in mass ratio) [0121] Dispersant:
polyoxyethylene secondary alkyl ether dispersant [0122] Fine
particles:
[0123] Silica (1): silica having an average particle diameter of
700 nm
[0124] Silica (2): silica having an average particle diameter of
200 nm
[0125] Silica (3): silica having an average particle diameter of
300 nm
[Preparation of Varnish-1]
[0126] Polyamide acid solution was obtained by blending 13.25 g of
the polyamide acid and 30 g of the organic solvent (1). 75 g of
silica (2) as the fine particles was added to the polyamide acid
solution thus obtained to prepare a first varnish. It should be
noted that a volume ratio of the polyamide acid and the silica (2)
in the first varnish was 22:78 (a mass ratio is 15:85).
[Preparation of Varnish-2]
[0127] Polyamide acid solution was obtained by blending 13.25 g of
the polyamide acid and 30 g of the organic solvent (1). 53 g of
silica (1) as the fine particles was added to the polyamide acid
solution thus obtained to prepare a second varnish. It should be
noted that a volume ratio of the polyamide acid and the silica (1)
in the second varnish was 28:72 (a mass ratio is 20:80).
[Formation of Uncalcined Composite Film]
[0128] A film of the second varnish was formed on a glass plate
with an applicator. An uncalcined composite film having a thickness
of 25 .mu.m was produced by pre-baking at 70.degree. C. for 5
minutes. The uncalcined composite film was peeled from the
substrate and dried to obtain an uncalcined composite film (1).
[Imidization of Uncalcined Composite Film]
[0129] The uncalcined composite film (1) was heated (post-baking)
at 320.degree. C. for about 15 minutes, to complete imidization and
obtain a polyimide-fine particle composite film (1).
[Formation of Porous Polyimide Film]
[0130] The polyimide-fine particle composite film (1) thus obtained
was immersed in 10% HF solution for 10 minutes to remove the fine
particles contained in the film, washed with water and dried, to
obtain a porous polyimide film (1).
[Formation of Two-Layered Uncalcined Composite Film]
[0131] A film of the first varnish was formed on a glass plate with
an applicator, and then baked at 70.degree. C. for 1 minute, to
obtain a first uncalcined composite film of about 2 .mu.m in
thickness. Subsequently, a second uncalcined composite film was
formed using the second varnish on the first uncalcined composite
film and pre-baked at 70.degree. C. for 5 minutes, to obtain a
two-layered uncalcined composite film (2) of about 25 .mu.m in
total thickness.
[Imidization of Uncalcined Composite Film]
[0132] The uncalcined composite film (2) was heated (post-baking)
at 320.degree. C. for about 15 minutes, to complete imidization and
obtain a polyimide-fine particle composite film (2).
[Formation of Porous Polyimide Film]
[0133] The polyimide-fine particle composite film (2) thus obtained
was immersed in 10% HF solution for 10 minutes to remove the fine
particles contained in the film, washed with water and dried, to
obtain a porous polyimide film (2).
Examples 1 to 8, and Comparative Examples 1 to 2
[0134] The porous polyimide film (2) thus obtained was subjected to
a comparison between different chemical etching conditions, using
an aqueous solution of tetraalkylammonium hydroxide (TMAH) or NaOH
solution, according to Table 1 below.
[Chemical Etching]
[0135] Alkaline etching solution was prepared by diluting: 2.38% by
mass aqueous solution of TMAH (referred to as "TMAH" in Table 1);
or NaOH (referred to as "NaOH" in Table 1) with 50% by mass aqueous
solution of methanol, to 1.04%. A part of polyimide was removed by
immersing the porous polyimide film in these etching solutions for
time periods specified in Table 1. FIG. 7 shows results of
observation by SEM of surfaces of the porous polyimide films
processed under conditions specified in Table 1.
Examples 9 to 12 and Comparative Examples 3 to 4
Gurley Permeability
[0136] The porous polyimide films (1) and (2) obtained above were
subjected to chemical etching according to Table 1 below, employing
the NaOH containing etching solution, which had exhibited, among
the results of Examples 1 to 8, superior form change of pores on
the surface upon removal of a part of the porous polyimide film;
and then samples of 25 .mu.m in thickness were cut therefrom in 5
cm squares. The time for 100 mL of air passing through the sample
was measured with a Gurley densometer (manufactured by Toyo Seiki
Co., Ltd.) in accordance with JIS P 8117. Results are also shown in
Table 1. Results of measurement of thickness by a contact probe
meter prior to and subsequent to the chemical etching are also
shown in Table 1 as pre-process thicknesses and post-process
thicknesses.
TABLE-US-00001 TABLE 1 Post- Pre-treatment treatment Porous Etching
Film Film Polyimide Etching Duration Thickness Thickness Permeation
Film Solution (min) (.mu.m) (.mu.m) Time (sec) Comparative (2) N/A
N/A Example 1 Example 1 (2) TMAH 0.5 Example 2 (2) TMAH 2 Example 3
(2) TMAH 5 Example 4 (2) TMAH 10 Comparative (2) N/A N/A Example 2
Example 5 (2) NaOH 0.5 Example 6 (2) NaOH 2 Example 7 (2) NaOH 5
Example 8 (2) NaOH 10 Comparative (1) N/A N/A 25.23 25.23 200
Example 3 Comparative (2) N/A N/A 25.56 25.56 170 Example 4 Example
9 (2) NaOH 0.5 25.92 25.26 100 Example 10 (2) NaOH 2 25.58 25.02 48
Example 11 (2) NaOH 5 25.12 23.78 40 Example 12 (1) NaOH 5 25.35
24.05. 45
[0137] FIG. 7 shows that a part of the porous polyimide film is
removed and the pore shape on the surface changes as a result of
chemical etching. In addition, the Gurley permeability of the
porous polyimide film in Table 1 shows that the time required for
100 ml of air to pass therethrough was substantially reduced as the
chemical etching proceeded, and communicability between a foreside
and a backside of the porous polyimide film was improved. From a
relationship between the permeation time and the thickness change,
the improvement in the permeability by the chemical etching is
thought to result not only from a reduction in thickness, but also
mainly from an increase in aperture ratio.
Example 13
[0138] A porous polyimide film was prepared in a similar way to
[Preparation of varnish] to [Formation of porous polyimide film]
described with respect to the porous polyimide film (2). After the
HF treatment, the porous polyimide film washed with water in a wet
state was placed on a polyethylene terephthalate (PET) film and
baked. Since a surface layer of the porous polyimide film was
electrostatically adhered to the PET film, by subsequently peeling
the porous polyimide film thus dried from the PET film, only the
surface layer remained on the PET film. It should be noted that a
face peeled from the PET film is the film formed by the first
varnish (a negative electrode side when used as a separator for a
lithium-ion battery).
[0139] FIG. 8 shows the surface of the PET film before and after
this treatment. The remaining segments of the porous polyimide film
surface layer can be observed on the surface of the PET film after
the treatment.
[0140] On the other hand, comparing the surface of the porous
polyimide film before the treatment with the surface of the porous
polyimide film after the treatment, it could be confirmed that the
surface layer of the porous polyimide film was removed by the above
described physical method, and pores were newly exposed to the
surface of the porous polyimide film.
Example 14
Preparation of Varnish-3
[0141] 6.5 g of tetracarboxylic dianhydride, 6.7 g of diamine, and
30 g of the organic solvent (1) were put into a separable flask
equipped with a stirrer, a stirring blade, a reflux condenser, and
a nitrogen gas inlet tube. Nitrogen was introduced into the flask
through the nitrogen gas introduction tube and a nitrogen
atmosphere was provided in the flask. Subsequently, the
tetracarboxylic dianhydride and the diamine were reacted at
50.degree. C. for 20 hours by stirring the contents of the flask to
prepare a polyamide acid solution. To the resulting polyamide acid
solution, 75 g of silica (3) having an average particle diameter of
300 nm was added such that a volume ratio between the polyamide
acid and the fine particles was 22:78 (mass ratio is 15:85),
followed by stirring to prepare a first varnish.
[Preparation of Varnish-4]
[0142] A second varnish in which a volume ratio is 28:72 (mass
ratio is 20:80) was prepared in a similar way to [Preparation of
varnish-3], except that 53 g of silica (1) having an average
particle diameter of 700 nm was added to the resulting polyamide
acid solution.
[Formation of Polyimide-Fine Particle Composite Film]
[0143] A film of the first varnish was formed on a glass plate with
an applicator, and then baked at 70.degree. C. for 1 minute, to
obtain a first uncalcined composite film of about 1 .mu.m in
thickness. Subsequently, a second uncalcined composite film was
formed using the second varnish on the first uncalcined composite
film and pre-baked at 70.degree. C. for 5 minutes, to obtain a
two-layered uncalcined composite film (2) of about 20 .mu.m in
total thickness.
[0144] The uncalcined composite film was peeled off from the
substrate, the release agent was then removed with ethanol, and a
heat treatment was performed at 320.degree. C. for 15 minutes to
complete imidization and obtain a polyimide-fine particle composite
film.
[Formation of Porous Polyimide Film]
[0145] The polyimide-fine particle composite film was immersed in a
10% HF solution for 10 minutes to remove the fine particles
contained in the film.
[Chemical Etching]
[0146] Alkaline etching solution was prepared by diluting 2.38% by
mass of an aqueous solution of TMAH with 50% by mass of an aqueous
solution of methanol, to 1.04%. A part of the polyimide surface was
removed by immersing the porous polyimide film in this etching
solution. Permeability of the porous polyimide film thus obtained
was 63 seconds.
Example 15
[0147] A porous polyimide film was formed as in Example 14 except
that the varnish was prepared by using a dispersant in an amount of
10 parts by weight based on 100 parts by weight of silica.
Permeability of the porous polyimide film thus obtained was 60
seconds.
[0148] The film characteristics of the porous polyimide films
prepared above were evaluated, and the results are summarized in
Table 2. The above described evaluation method for Gurley
permeability was used.
[Tensile Strength]
[0149] In order to evaluate the strength of a porous polyimide
film, the tensile strength of the porous polyimide film was
measured. The porous polyimide films of Examples 14 and 15 were
each cut into 1 cm.times.5 cm strip samples. The stress (MPa) at
the time when this sample was broken was evaluated with RTC-1210A
TENSILON (manufactured by ORIENTEC Co., Ltd.).
TABLE-US-00002 TABLE 2 Per- Film meation Tensile Chemical Thickness
Time Stress Dispersant Etching (.mu.m) (sec) (MPa) Example 14
Absent Present 20 63 7 or higher Example 15 Present Present 20 60 7
or higher
[0150] Example 15 with the dispersant shows a shorter permeation
time than Example 14 without the dispersant, and therefore
improvement in interconnectivity between pores. In addition, the
strength of the film did not deteriorate even after the chemical
etching, and the film had good handleability.
<Production of Coin Battery for Evaluation>
[0151] In a coin outer container made of stainless steel and having
a diameter of 20 mm, a carbon negative electrode, a separator of
Examples 14 and 15 cut into a circle having a diameter of 14 mm,
metal lithium cut into a circle having a diameter of 14 mm, and a
spacer of copper foil cut into a circle having a diameter of 14 mm
and a thickness of 200 mm were stacked in this order; several drops
of an electrolytic solution (1 moldm.sup.-3 of LiPF.sub.6: a
solution mixture of ethylene carbonate/diethyl carbonate=1/1
(volume ratio)) were added to the container so as not to overflow
from the container; and the container was capped with a stainless
steel cap via polypropylene packing and was sealed with a caulking
tool for producing coin batteries to produce a battery for
evaluating a separator. Upon production, the separator was used
such that a surface thereof with the layer which had been produced
by using the first varnish faced the negative electrode. The
batteries were used as Examples B1 and B2.
<Evaluation of Charge-Discharge Characteristics of Coin
Battery>
[0152] The charge-discharge characteristics were evaluated by
charging each coin battery for evaluation at a current density of
2.2 mAh (1C) up to 4.1 V (CC-CV operation) and then discharging the
battery at a current density of 2.2 mAh (1C) or 3C down to 2.5 V
(CC operation), in a thermostatic chamber. Table 3 shows the
results. In Table 3, the values shown in parentheses are the
electrostatic capacity retention ratios (%) at a rate of 3C when
the capacity at 1C is defined as 100%.
<Production of Monolayer Laminate Cell Battery for
Evaluation
>
[0154] A positive electrode of 20 mm.times.20 mm and a separator of
20 mm.times.20 mm of the respective above-described Examples were
placed in an aluminum laminate outer container in this order, and
an electrolytic solution (solvent: ethylene carbonate:ethyl methyl
carbonate=3:7, electrolyte salt: 1 mol/l of LiPF.sub.6) was added
thereto. Furthermore, a negative electrode of 20 mm.times.20 mm was
placed in the container, and the battery case was sealed to obtain
lithium ion secondary batteries of Examples B1 and B2. Here, a
nickel-cobalt-manganese ternary positive electrode and an
artificial graphite negative electrode were used as the electrodes,
and arranged such that the surface of the layer formed of the first
varnish was in contact with the negative electrode.
[0155] Furthermore, monolayer laminate cell batteries were prepared
similarly to the above except that a commercially available
polyethylene-based (PE-based) or cellulose-based separator was
used. These batteries were used as Comparative Examples B1 and B2.
The PE-based separator being used had an average pore diameter of
80 nm, a thickness of 20 .mu.m, an air permeability of 270 seconds,
and a porosity of 42%; and the cellulose-based separator being used
had an average pore diameter of 3000 nm, a thickness of 25 .mu.m,
an air permeability of 135 seconds, and a porosity of 70%.
<Charge-Discharge Characteristics of Monolayer Laminate Cell
Battery>
[0156] Using the resulting lithium ion secondary battery, the
potential change by lithium occlusion was measured with a
charge-discharge measuring apparatus. The battery was charged up to
4.2 V at a charging speed of 0.2C at 25.degree. C., and after a
pause of 10 minutes, was then discharged at a discharging speed of
2C down to a voltage range of 2.7 V. After the discharge, a pause
of 10 minutes was taken. The battery was evaluated for the Ah
utilization rate and Wh utilization rate (energy retention ratio)
during this process. Results are shown in Table 3.
<Heat Resistance of Separator>
[0157] The separator used in each battery was evaluated for the
heat resistance using a soldering iron of about 250.degree. C.
according to the following criteria:
[0158] .largecircle.: When a tip of the soldering iron is pressed
against the center of a film, a mark was left, but the film was not
broken.
[0159] X: When a tip of the soldering iron is pressed against the
center of a film, the film was pierced.
<Crushing Test of Monolayer Laminate Cell Battery>
[0160] A crushing test was performed by charging a monolayer
laminate cell battery at a voltage of 4.2 V and then compressing
the battery in a resting state in the direction vertical to the
length direction with a round bar having a diameter of 15.8 mm. The
time when the voltage reduced was determined as the occurrence of
internal short-circuit of the battery, and the battery was
evaluated by the pressure at the time of the occurrence of the
internal short-circuit. The amount of reduction in voltage 5
seconds after the time of the reduction of the voltage was defined
as .DELTA.V (V). A higher value of the pressure is preferred, and a
lower reduction in voltage is preferred.
TABLE-US-00003 TABLE 3 Coin Battery Monolayer Laminate Cell Battery
Evaluation Results Evaluation Ah Wh Results Utilization Utilization
Crushing Test Separator 1 C 3 C Rate Rate Heat Pressure .DELTA.V
used (mAh) (mAh) (%) vs 0.2 C (%) vs 0.2 C Resistance (ton) (after
5 sec) Example B1 Example 2.2 1.7 88.2 84.4 .smallcircle. 0.57 0.26
14 (76.5%) Example B2 Example 2.2 1.7 88.1 84.2 .smallcircle. -- --
15 (75.4%) Comparative PE -- -- 83.8 78.7 x -- -- Example B1
Comparative Cellulose -- -- 87.6 83.9 .smallcircle. 0.47 0.56
Example B2
<Charge-Discharge Characteristics of Monolayer Laminate Cell
Battery: Low Temperature and Increase in Charging Speed>
[0161] For the lithium ion secondary batteries produced in Example
B1 and Comparative Examples B1 and B2, the potential change due to
lithium occlusion was measured by using a charge-discharge
measuring apparatus. At a temperature of 0.degree. C. and a
charging speed of 1C, an Ah efficiency and a capacity retention
ratio at a fifth cycle (ratio to the electric discharge capacity at
a first cycle) were obtained under conditions of: charging CCCV:
4.2 V; CV: 1 hour; discharge: 2.7 V. Results are shown in Table
4.
<Observation of Negative Electrode Surface after Evaluation of
Charge-Discharge Characteristics>
[0162] The negative electrode was taken out from the battery after
completion of the above described charge-discharge characteristics
evaluation test, and a surface thereof was observed. Statuses of
dendrite generation on the negative electrode surfaces are shown in
Table 4. In addition, the negative electrode surfaces of Example
B1, Comparative Example B1 and Comparative Example B2 were enlarged
500 times and observed with an optical microscope. The images are
shown in FIGS. 9 to 11.
TABLE-US-00004 TABLE 4 Monolayer Laminate Cell Battery Evaluation
Results Capacity Ah Retention Lithium Dendrite Separator Efficiency
Ratio on Negative used (%) (%) Electrode Example B1 Example 99.82
98.73 Slight whiteness 14 observed on 4 corners Comparative PE
95.36 77.69 Whiteness observed Example B1 in outer periphery
Comparative Cellulose 99.68 98.63 Whiteness observed Example B2 in
outer periphery
[0163] Operation of a coin battery and a monolayer laminate cell
battery employing the separator of Example of the present invention
was confirmed. In addition, it was confirmed that the battery of
Example B1 employing the separator of the present invention was
superior in heat resistance and performance in the crushing test to
Comparative Examples B1 and B2 employing commercially available
separators.
[0164] FIGS. 9 to 11 are pictures of surfaces of negative
electrodes taken out from batteries of Example B1 and Comparative
Examples B1 and B2 after completion of the above described
charge-discharge characteristics evaluation test. White bright
spots due to generation of lithium dendrite were observed on each
of these. In the battery of Example B1, there are only a few bright
spots on the negative electrode, showing that lithium dendrite
generation due to charge and discharge is effectively suppressed.
On the contrary, the bright spots on the negative electrodes taken
out from Comparative Examples B1 and B2 are large in size and
number. In other words, it can be observed that lithium dendrite
generation occurs more frequently in Comparative Examples employing
commercially available separators based on polyethylene (PE) or
cellulose, compared to a case of using the separator of the present
invention.
[0165] Furthermore, Table 4 and FIGS. 9 to 11 show that the battery
employing the separator of Example of the present invention can
suppress an amount of lithium dendrite generated on a negative
electrode surface and provides a superior capacity retention ratio
at low temperature, suggesting that an electrical field applied to
the separator is homogenized.
<Observation of Negative Electrode Surface after Evaluation of
Charge-Discharge Characteristics>
[Preparation of Varnish-5]
[0166] 20 g of polyamide imide, 80 g of silica (1) as the fine
particles, 0.4 g of dispersant, and the organic solvent (2) were
blended and stirred to prepare a varnish (5). It should be noted
that a mass ratio of polyamide imide to silica (1) in the varnish
(5) was 20:80, and a volume ratio was approximately 28:72.
[Formation of Uncalcined Composite Film]
[0167] A film of the varnish (5) was formed on a PET film with an
applicator. An uncalcined composite film having a thickness of
approximately 28 .mu.m was produced by pre-baking at 100.degree. C.
for 5 minutes. The uncalcined composite film was peeled from the
substrate and dried to obtain an uncalcined composite film (5),
that is a polyamide imide-fine particle composite film (1). From
the uncalcined composite film (5) (polyamide imide-fine particle
composite film (1)), the solvent had been removed. The uncalcined
composite film was not subjected to a calcining step.
[Formation of Porous Polyamide Imide Film]
[0168] The polyamide imide-fine particle composite film (1) thus
obtained was immersed in 10% HF solution for 10 minutes to remove
the fine particles contained in the film, washed with water and
dried, to obtain a porous polyamide imide film (1).
Examples 16 to 19
[0169] The porous polyamide imide film (1) thus obtained was
subjected to a comparison between different chemical etching
conditions, using NaOH solutions described below, according to
Table 5 below. [0170] Chemical etching solution A: NaOH being
diluted with 30% by mass ethanol aqueous solution
(H.sub.2O:EtOH=70:30) to 1.0% by mass [0171] Chemical etching
solution B: NaOH being diluted with 30% by mass ethanol aqueous
solution (H.sub.2O:EtOH=70:30) to 1.5% by mass [0172] Chemical
etching solution C: NaOH being diluted with 30% by mass isopropanol
aqueous solution (H.sub.2O:IPOH=70:30) to 1.0% by mass [0173]
Chemical etching solution D: NaOH being diluted with 30% by mass
isopropanol aqueous solution (H.sub.2O:IPOH=70:30) to 1.5% by
mass
[Chemical Etching]
[0174] A part of polyimide was removed by immersing the porous
polyamide imide film in the above described etching solutions A to
D for 2 minutes (at ambient temperature). Film thicknesses prior to
and subsequent to the chemical etching, as well as results of
Gurley permeability after the chemical etching (Gurley permeability
before the chemical etching for Comparative Example 5) are shown in
Table 5.
TABLE-US-00005 TABLE 5 Pre-treatment Film Post-treatment Gurley
Etching Thickness Film permeability Solution (.mu.m) Thickness
(.mu.m) (sec) Comparative Absent 28.0 -- 135 Example 5 Example 16 A
28.1 27.9 93.6 Example 17 B 28.7 27.4 87.1 Example 18 C 27.6 24.9
108.3 Example 19 D 26.9 25.0 109.6
[0175] Table 5 shows that, similarly to the case of the porous
polyimide film, even in a case of employing polyamide imide as the
polyimide resin, the chemical etching improved Gurley permeability
and communicability between a foreside and a backside of the porous
polyamide imide film. From a relationship between the permeation
time and the thickness change, the improvement in the permeability
by the chemical etching is thought to result from not only a
reduction in thickness, but also mainly from an increase in
aperture ratio as a result of enlargement of openings on the
surface and interconnected pores.
EXPLANATION OF REFERENCE NUMERALS
[0176] 1 Fine particles [0177] 2 Polyimide resin portion [0178] 3
Pores [0179] 4 Porous polyimide resin film
* * * * *